Thermal analysis by TGA is used as a tool to investigate the inter-relationships between thermal degradation properties of pure polymer plastics and those of their mixtures. Thermal decomposition of plastic mixtures comprising high density polyethylene, polystyrene and polypropylene were examined under nitrogen atmosphere. The results show that if mixed, the individual components are more clearly recognised from the differential mass loss curves. Weighed sum method was applied to evaluate the share of the constituents. Inaccuracy in determining the composition could be caused by interaction between constituents. To illuminate the effect of interaction, an equation describing the difference of weight loss between experimental and theoretical ones is defined. In addition, activation energy was calculated by model free method and the dependencies of activation energy on conversion were studied.